1. Technical Field
The present disclosure relates to a reaction vessel for a fuel cell, and more particularly to a reaction vessel with improved thermal efficiency, and a reaction device for a steam reforming reaction.
2. Discussion of Related Art
A fuel cell is a power generation system for generating electricity by electrochemically reacting hydrogen with oxygen. Fuel cells are divided into phosphate fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer-electrolyte-membrane fuel cells, alkaline fuel cells, etc., depending on the type of electrolyte used. Each of these types of fuel cell operates on the same basic principle, but the type of fuel, operating temperature, catalyst, electrolyte, etc. are different. Among these types of fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) exhibits excellent output characteristics, a low operating temperatures, and rapid start-up and response times, compared to the other types of fuel cells, and are widely used in distributed electric power sources that can be stationary, for example, for houses or public facilities, or mobile, for example, for portable electronic equipment or automobiles.
Hydrogen has best reactivity as the fuel for the electrochemical oxidation reaction that occurs at the anode electrode of a fuel cell, where it reacts with oxygen to generate water. Furthermore, hydrogen does not generate any air pollutants. However, pure hydrogen gas is typically generated by reforming other compounds because does not a naturally occur in its elemental form on earth. For example, hydrogen may be obtained from a hydrocarbon-based fuel such as gasoline, diesel, methanol, ethanol, natural gas, etc. through a reforming. Also, hydrogen may also be easily obtained from fuel sources such as commercially available butane cans. Accordingly, if a butane can is used as a fuel source for a fuel cell, the fuel may be conveniently supplied using the internal pressure of the butane can.
Some fuel cells have a reformer that generates hydrogen from a hydrocarbon-based fuel. The inner part of the reformer may be divided into a reforming reaction unit, a water-gas shift unit, and a preferential oxidation unit to further enhance the reforming efficiency. The reforming reaction unit may be further divided into a steam reforming reaction unit and an autothermal reforming reaction unit.
Each of the reaction units in the reformer has a different reaction temperature range according to the reaction catalyst used therein. For example, the reaction temperature range of the steam reforming (SR) reaction varies according to the feedstock. Here, if the feedstock is a hydrocarbon-based fuel such as butane, the reaction temperature of the steam reforming reaction ranges from about 600° C. to about 900° C., but if the feedstock is methanol, the reaction temperature of the steam reforming reaction ranges from about 250° C. to about 400° C. And, the reaction temperature range of the water gas shift (WGS) reaction, which is one of the processes for removing carbon monoxide, ranges from about 200° C. to about 350° C., while the reaction temperature range of a preferential CO oxidation (PROX) reaction ranges from about 100° C. to about 250° C. As described above, the reaction temperature range of each of the reaction units in the reformer goes down in the following order: the reforming reaction unit, the water gas shift unit, and the preferential oxidation unit.
In order to provide the desired reaction temperature range for each of the units, a heat source, for example, an electric heater such as nichrome wire or a device that burns a hydrocarbon fuel, optionally, using a catalyst.
The steam reforming reaction unit converts steam and a hydrocarbon fuel into hydrogen molecules and carbon dioxide at high temperatures, as shown in Scheme 1. Butane is used as the hydrocarbon in the following examples. The reaction illustrated in Scheme 1 is an endothermic reaction because high-energy state hydrogen molecules are generated from low energy state water molecules. A large amount of hydrogen is generated in the endothermic reaction, however.

In the autothermal reforming reaction unit, a hydrocarbon reacts with oxygen from the air at the steam reforming reaction temperature to provide hydrogen molecules and carbon dioxide as shown in Scheme 2. The reaction is exothermic because the carbon atoms are oxidized and none of the hydrogen atoms in the product come from water molecules as in the steam reforming reaction.

In a steam reforming reaction or an autothermal reforming reaction, carbon dioxide should be theoretically generated, but in practice, a large amount of carbon monoxide is actually generated due to incomplete reactions. As one of the devices for reducing carbon monoxide, the water-gas shift unit reacts carbon monoxide with steam to completely oxidize the carbon monoxide to carbon dioxide, and simultaneously generate hydrogen molecules are from the hydrogen atoms of the water molecules. The water-gas shift reaction is shown in Scheme 3.

As another one of the devices for reducing carbon monoxide, the preferential oxidation unit oxidizes carbon monoxide to carbon dioxide by reacting the carbon monoxide with oxygen from the air.

The reforming efficiency of the reformer is large part of the total efficiency of the fuel cell system. To improve the efficiency of the reformer, the components in the reformer, such as the reforming reaction unit, the water gas shift unit, and the preferential oxidation unit, in which the chemical reactions occur, should be rapidly heated their respective reaction temperatures. Efficiency can also be improved when each unit in the reformer has a high surface area structure on which the reaction catalyst disposed and which contacts the reactant gas phase the desired temperature.
It is also desirable to increase the actual contact area between the catalyst and the gas using a reactor with a structure that promotes mixing of the gas.
Furthermore, the heat energy used in the reforming process is discharged out of the reformer, which wastes the heat energy, and therefore, reduces the overall electric generation efficiency of the system.